Most batteries currently utilize liquid electrolytes to carry ions between the anode and cathode. In lithium-ion batteries, these electrolytes most commonly comprise organic solvents, lithium salts, and performance-enhancing additives. Typical electrolytes in common use comprise carbonate solvents such as ethylene carbonate (EC), diethylcarbonate (DEC), dimethylcarbonate (DMC), and propylene carbonate (PC). Among the conducting salts lithium hexafluorophosphate (LiPF6) is used almost exclusively as conducting salt for state-of-art high energy lithium-ion batteries[1]. Nevertheless, its limited thermal and chemical stability restricts its use at low and high temperatures. In the carbonate-based solvents, the poorly solvated PF6− anion is highly reactive towards even weak nucleophiles and the presence of even a trace amount of water (or alcohols) produces hydrofluoric acid [2]-[5]. Also at high temperature, LiPF6 decomposes to produce phosphorous pentafluoride [LiPF6=LiF+PF5][6], [7] PF5 which in turn reacts with solvent to generate highly toxic substances[8] or initiate polymerization of solvents[3], [7]. This raises the safety issues particular of the use in lithium ion batteries for passenger cars. Therefore, new lithium salts with improved properties are highly desired to improve the safety and performance of lithium-ion batteries.
Over past two decades, great efforts have been made to develop new lithium salts with improved chemical and/or electrochemical properties. Thus, various weakly coordinated anions have been proposed as possible counter parts of lithium salts for Li-ion batteries. Among those, lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) and lithium bis(fluorosulfonyl)imide (LiFSI) salts have shown no spontaneous decomposition and hydrolytically are more stable than LiPF6. LiTFSI was initially suggested by Armand and Kadiri [9] then commercialized by 3M. However, LiTFSI has the disadvantage of being corrosive to aluminum current collectors[10] which is not the case with LiFSI. Therefore, LiFSI is a promising alternative salt for lithium ion batteries and ultra-capacitors.
Very recently, there has been growing interest in LiFSI as a conducting salt. LiFSI was first claimed as conducting salts with good anti-corrosive properties in Li-ion batteries in 1995[11] however, a little attention was paid to it until recently[12] presumably due to the fact that it is difficult to prepare with high purity in an unspecialized laboratory.
To prepare an electrolyte, solvents, salts and additives are mixed together. Since the water content of the final electrolyte must be very low (typically 500 ppm or less) either this mixing must be conducted under anhydrous conditions, or water must be removed from the electrolyte after one or more of the components are mixed together. The requirement for conducting these operations without introducing water into the electrolyte increases the cost and complexity of preparing electrolytes. Water may be removed from the complete electrolyte by various methods. The use of molecular sieves or zeolites to remove water from organic solvents is well-known. However, the zeolites used typically contain other metals that are not lithium, and may exchange with lithium-ions in solution, both reducing the lithium content of the electrolyte and increasing the metal impurities in the electrolyte. Lithium-exchanged zeolites such as lithium beta zeolite sold by Tosoh may be used, but these zeolites are substantially more expensive than sodium zeolites and are not easily regenerated.
The preparation of electrolytes is further complicated by the need to handle dry powders of the lithium salts. Many lithium salts, including LiPF6 and LiFSI, are highly hygroscopic. LiFSI is deliquescent; it will absorb sufficient moisture from the air to form a liquid solution. For these reasons, lithium salts are supplied as a powder in sealed containers with complex airlocks to connect with mixing equipment without introducing moisture. As the material must be handled as a powder, the salt must be prepared in a form that is free-flowing. Many disclosed techniques for preparing lithium salts produce a fine powder that is not free-flowing. To convert the crude lithium salts to a free-flowing powder requires additional manufacturing steps such as agglomeration or slow crystallization. Each of these steps requires additional time, capital costs and operating costs and reduces the yield of the salt.
In some cases, it may be possible to avoid the recrystallization and isolation steps by preparing the lithium salt directly in the final electrolyte solvent. For example U.S. Pat. No. 5,496,661, issued to Mao, discloses the preparation of a 1 molar solution of LiPF6 in a mixture of EC and DEC by the reaction of NH4PF6 and LiH in the solvents, followed by removal of the product gases and filtration of solid impurities. While these methods avoid the crystallization and handling of solid lithium salts, the resulting electrolyte is generally of lower purity than that produced by mixing recrystallized LiPF6 salts and highly pure solvents. Thus the use of these electrolytes is generally restricted to primary cells and lower-performance batteries.
In addition, LiPF6 has limited solubility in carbonate solvents and electrolytes with concentrations above 1 molar crystallize or form glasses upon cooling below room temperature, which presents problems in shipping and handling. Due to the limited solubility, it is not practical to prepare a master solution of concentrated lithium salt and solvent which could be diluted to provide a variety of electrolyte formulations.
Thus, state-of-the-art methods for preparing lithium salts for use in electrolytes generally involve crystallization and drying steps to produce free-flowing powders. Electrolyte preparation requires handling these powders without introducing water, which increases the cost and complexity of electrolytes. Therefore, there is a clear unmet need for simplified methods of delivering lithium salts for electrolytes that avoids the need to handle solids and provides an electrolyte of high purity.